<!--
 * @Description: ***
 * @Version: 1.0.0
 * @Autor: Caoyingwen
 * @Date: 2022-01-05 17:42:27
 * @LastEditors: Caoyingwen
 * @LastEditTime: 2022-01-05 18:08:14
-->
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>demo02</title>
</head>
<body>
  <canvas width = "300" height = "300" id = "my_Canvas"></canvas>
		
  <script>         
  /* Step1: Prepare the canvas and get WebGL context */

     var canvas = document.getElementById('my_Canvas');
     var gl = canvas.getContext('experimental-webgl');         /* Step2: Define the geometry and store it in buffer objects */

     var vertices = [-0.5, 0.5, -0.5, -0.5, 0.0, -0.5,];

     // Create a new buffer object
     var vertex_buffer = gl.createBuffer();

     // Bind an empty array buffer to it
     gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);
     
     // Pass the vertices data to the buffer
     gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);

     // Unbind the buffer
     gl.bindBuffer(gl.ARRAY_BUFFER, null);         /* Step3: Create and compile Shader programs */

     // Vertex shader source code
     var vertCode =
        'attribute vec2 coordinates;' + 
        'void main(void) {' + ' gl_Position = vec4(coordinates,0.0, 1.0);' + '}';

     //Create a vertex shader object
     var vertShader = gl.createShader(gl.VERTEX_SHADER);

     //Attach vertex shader source code
     gl.shaderSource(vertShader, vertCode);

     //Compile the vertex shader
     gl.compileShader(vertShader);

     //Fragment shader source code
     var fragCode = 'void main(void) {' + 'gl_FragColor = vec4(0.0, 0.0, 0.0, 0.1);' + '}';

     // Create fragment shader object
     var fragShader = gl.createShader(gl.FRAGMENT_SHADER);

     // Attach fragment shader source code
     gl.shaderSource(fragShader, fragCode);

     // Compile the fragment shader
     gl.compileShader(fragShader);

     // Create a shader program object to store combined shader program
     var shaderProgram = gl.createProgram();

     // Attach a vertex shader
     gl.attachShader(shaderProgram, vertShader); 
     
     // Attach a fragment shader
     gl.attachShader(shaderProgram, fragShader);

     // Link both programs
     gl.linkProgram(shaderProgram);

     // Use the combined shader program object
     gl.useProgram(shaderProgram);         /* Step 4: Associate the shader programs to buffer objects */

     //Bind vertex buffer object
     gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);

     //Get the attribute location
     var coord = gl.getAttribLocation(shaderProgram, "coordinates");

     //point an attribute to the currently bound VBO
     gl.vertexAttribPointer(coord, 2, gl.FLOAT, false, 0, 0);

     //Enable the attribute
     gl.enableVertexAttribArray(coord);         /* Step5: Drawing the required object (triangle) */

     // Clear the canvas
     gl.clearColor(0.5, 0.5, 0.5, 0.9);

     // Enable the depth test
     gl.enable(gl.DEPTH_TEST); 
     
     // Clear the color buffer bit
     gl.clear(gl.COLOR_BUFFER_BIT);

     // Set the view port
     gl.viewport(0,0,canvas.width,canvas.height);

     // Draw the triangle
     gl.drawArrays(gl.TRIANGLES, 0, 3);
     

</script>
</body>
</html>